The overall aim of the application is to characterize cellular transcription factors which are involved in basal and tax-mediated transactivation of the HTLV-1 LTR. Specific aims include (1) characterizing the interaction of SP and ATF/CREB proteins with tax and promoter elements in the LTR and impact on LTR activity. (2) Examine interaction of AP-1 and ATF/CREB proteins with tax and promoter elements in the LTR. (3) Examine the role of the tax response element in basal and tax-mediated LTR activity. (4) Examine properties of HTLV-1 variants containing mutations in specific promoter regions which disrupt interaction with SP:ATF/CREB and AP:ATF/CREB protein complexes.